In optical communications, optical signals are encoded with digital information at a transmitter site, propagated towards a receiver site, and decoded at the receiver site. The optical signals are encoded by modulating amplitude and/or phase of the signals. Phase modulation generally allows for a better bandwidth utilization. However, phase modulated signals are more difficult to decode, because the decoding requires a transformation of phase modulation into amplitude modulation that can be detected. To transform the phase modulation into the amplitude modulation, a reference optical signal is mixed with the transmitted phase-modulated signal in an optical interferometer. The amplitude-modulated interference signal is then detected by a photodetector.
One can use the optical signal itself as the reference optical signal. In a so-called differential phase shift keying (DPSK) interferometer, a transmitted optical signal is split into two portions of equal amplitude, one portion is delayed relative to the other portion by a time delay corresponding to duration of one bit of information, and the two portions are combined to provide an optical interference signal. Referring to FIG. 1A, a planar lightwave circuit (PLC) DPSK interferometer 100 of prior art includes a 1×2 waveguide splitter 102, upper and lower waveguides 104 and 105, respectively, and a 2×2 optical coupler 106. A phase-modulated optical signal 108 is coupled to an input port 101 of the DPSK interferometer 100. The optical signal 108 is split by the 1×2 waveguide splitter 102 into two portions 109 and 110 of equal amplitude, which propagate in the upper and lower waveguides 104 and 105, respectively. Since the upper waveguide 104 is longer than the lower waveguide 105, the portion 109 will be delayed relative to the portion 110. The lengths difference of the upper and lower waveguides 104 and 105 is selected so as to delay the portion 109 by one bit duration of the phase-modulated signal 108. The two portions 109 and 110 interfere with each other in the 2×2 optical coupler 106. A differential photodetector pair 114 is coupled to output waveguides 111 and 112 of the 2×2 optical coupler 106 to detect an interference signal.
Turning now to FIG. 1B, the optical signal portions 109 and 110 are illustrated by means of phase and amplitude time diagrams 121 and 122; 123 and 124, respectively. The lower phase diagram 123 illustrates the optical signal portion 109, which is the portion 110 delayed by one bit. An amplitude time diagram 125 illustrates time dependence of the interference signal's amplitude. When phases of the optical signal portions 109 and 110 are equal, the amplitude is equal to 1.0 due to constructive interference, and when the phases are opposite (that is, one is π, and the other one is 0), the amplitude is equal to 0 due to destructive interference. The amplitude-modulated signal 125 can be detected by the differential photodetector pair 114.
By using four values of phase, 0, π/2, π, and 3π/2, one can further improve bandwidth utilization of optical phase modulation. This variety of DPSK modulation is termed differential quadrature phase shift keying (DQPSK) modulation. Referring to FIG. 2, a prior-art DQPSK interferometer 200 is shown. The DQPSK interferometer 200 includes the 1×2 splitter 102 coupled to two DPSK interferometers 201 and 202 having branch waveguides 204, 205, 206, and 207 providing phase delays of 0, π/2, π, and 3π/2, respectively, the long branch waveguides 204 and 206 providing additional one-bit delays relative to the short branch waveguides 205 and 207. Phase shifters, not shown, are used to fine tune phase delays in the branch waveguides 204, 205, 206, and 207. Two differential photodetector pairs 114 are used to detect the optical interference signals.
The PLC-based DPSK and DQPSK interferometers 100 and 200 share a common drawback of temperature dependence of the phase shifts, as well as polarization sensitivity. Complicated temperature control is usually required to achieve a reliable and stable operation of PLC devices. In U.S. Pat. No. 7,259,901 by Parsons et al., PLC interferometers of different branch length, one providing a delay slightly bigger than a bit delay, and the other providing a delay slightly smaller than the bit delay, are used to tune a PLC interferometer by uniformly varying temperature of the entire PLC chip. In US Patent Application Publication US2007/0177151 by Isomura et al., a separate heating elements and a highly conductive, thermally matched spacer are used to control temperature of the PLC chip with precision required for stable and reliable phase demodulation.
A free-space Michelson interferometer, which is not as sensitive to temperature as the PLC interferometers 100 and 200, can be used for DPSK demodulation of optical signals. Referring now to FIG. 3, a Michelson interferometer 300 includes a half mirror 302 having 50% reflectance and first and second mirrors 304 and 306 spaced from the half-mirror 302 by distances L and L+cΔt/2, where Δt is bit duration and c is speed of light. A free space optical beam 301, carrying the phase-modulated signal, impinges on the half-mirror 302. Output interference signals 308 and 310 are detected by separate photodetectors, not shown. Michelson interferometer DPSK/DQPSK demodulators are known. By way of example, Michelson interferometer DPSK/QPSK demodulators have been disclosed in U.S. Pat. No. 7,411,725 by Suzuki et al. and in U.S. Pat. Nos. 7,489,874 and 7,526,210 by Liu. Detrimentally, Michelson interferometers of the prior art tend to be bulky and have a slow phase delay adjustment time as compared to their PLC counterparts. Furthermore, the prior-art PLC interferometers 100 and 200, and the prior-art Michelson interferometer 300 art can only operate at a single fixed bit rate of a phase-modulated optical signal. A different interferometer is required to operate at a different bit rate.
It is a goal of the present invention to provide a DPSK/DQPSK interferometer that would combine a compact size, a good thermal stability, and quick phase delay adjustment time with an option to adjust or switch the bit delay for operation at different bit rates.